Abstract

   Periodontal disease is of established etiology in which polymicrobial
   synergistic ecology has become dysbiotic under the influence of
   Porphyromonas gingivalis. Following breakdown of the host's protective
   oral tissue barriers, P. gingivalis migrates to developing inflammatory
   pathologies that associate with Alzheimer's disease (AD). Periodontal
   disease is a risk factor for cardiovascular disorders (CVD), type II
   diabetes mellitus (T2DM), AD and other chronic diseases, whilst T2DM
   exacerbates periodontitis. This study analyzed the relationship between
   the P. gingivalis/host interactome and the genes identified in
   genome-wide association studies (GWAS) for the aforementioned
   conditions using data from GWASdb (P < 1E-03) and, in some cases, from
   the NCBI/EBI GWAS database (P < 1E-05). Gene expression data from
   periodontitis or P. gingivalis microarray was compared to microarray
   datasets from the AD hippocampus and/or from carotid artery plaques.
   The results demonstrated that the host genes of the P. gingivalis
   interactome were significantly enriched in genes deposited in GWASdb
   genes related to cognitive disorders, AD and dementia, and its
   co-morbid conditions T2DM, obesity, and CVD. The P. gingivalis/host
   interactome was also enriched in GWAS genes from the more stringent
   NCBI-EBI database for AD, atherosclerosis and T2DM. The misregulated
   genes in periodontitis tissue or P. gingivalis infected macrophages
   also matched those in the AD hippocampus or atherosclerotic plaques.
   Together, these data suggest important gene/environment interactions
   between P. gingivalis and susceptibility genes or gene expression
   changes in conditions where periodontal disease is a contributory
   factor.

   Keywords: Alzheimer's disease, cardiovascular, diabetes, interactome,
   Porphyromonas gingivalis

Introduction

   Complex chronic diseases such as periodontitis, cardiovascular disease
   (CVD; including atherosclerosis, strokes, hypertension, myocardial
   infarction, congestive heart failure), type 2 diabetes mellitus (T2DM)
   and Alzheimer's disease (AD), are increasingly common during advanced
   aging and hence place a considerable social and economic burden
   globally. Porphyromonas gingivalis is a risk factor for periodontitis
   and is associated with distal inflammatory pathologies including CVD,
   T2DM, and AD via immune modification mechanisms. The prevalence of both
   periodontitis and AD increases with aging (Silvestre et al., [29]2017)
   as do CVD (Qiu and Fratiglioni, [30]2015) and T2DM (Yakaryilmaz and
   Ozturk, [31]2017). The aging process itself may contribute to these
   conditions, particularly in relation to pathogens, via
   immunosenescence. This can decrease resistance to pathogens due to
   immunodeficiency but is also accompanied by an increase in the
   pro-inflammatory activity of monocytes and macrophages which can lead
   to chronic low grade, inflammation, termed “inflammageing,” which is
   also associated with AD, CVD, and T2DM (Fülöp et al., [32]2016).

   Periodontitis is of particular interest because of its known
   polymicrobial etiology and a recognized oral microbiome (Socransky et
   al., [33]1998; Aas et al., [34]2005). The Human Oral Microbiome
   Database [35]www.homd.org (Chen et al., [36]2010). An uncontrolled oral
   microbiome may act as a reservoir, from which opportunistic pathogens
   can migrate to remote body organs. P. gingivalis and oral spirochetes
   for example, associate with extra-oral niches (Riviere et al.,
   [37]2002; Poole et al., [38]2013; Olsen and Progulske-Fox, [39]2015).
   Periodontal pathogens enter the systemic system through daily
   bacteraemia following breakdown of epithelial: endothelial barriers due
   to the host's inflammatory responses and the ability of some pathogens,
   including P. gingivalis to attack these barriers (Katz et al.,
   [40]2000).

   Chronic periodontitis is associated with the sporadic form of AD and
   other related comorbidities, T2DM, and atherosclerosis inter alia (Löe,
   [41]1981, [42]1993; Olsen and Singhrao, [43]2015; Singhrao et al.,
   [44]2015; Olsen et al., [45]2016; Bale et al., [46]2017; Harding et
   al., [47]2017). Here our focus is on P. gingivalis as the keystone
   pathogen, because it is by far the best-researched bacterium for its
   contribution to periodontitis (Hajishengallis et al., [48]2012;
   Hajishengallis and Lamont, [49]2014; Olsen et al., [50]2016).

   Periodontal disease is an inflammatory condition affecting the tissues
   supporting teeth in their bony socket and occurs in aggressive and
   chronic subtypes. The disease is caused by polymicrobial dysbiosis with
   several bacterial species playing a significant role in tooth loss
   (Hajishengallis and Lamont, [51]2014; Olsen et al., [52]2016). Some of
   these include Actinomycetem actinomycetemcomitans, and those belonging
   to the red complex (P. gingivalis, Tannerella Forsythia, Treponema
   denticola). Others are intermediate colonizers of the sub-gingival
   biofilm ecology of the orange complex, (Fusobacterium nucleatum,
   Peptostreptococcus micros, Prevotella intermedia, Prevotella nigrecens,
   Eubacterium nodatum, and Streptococcus constellates) (Haffajee et al.,
   [53]2008) and other species. Together they contribute to pathological
   periodontal pocket formation around the tooth.

   The importance of P. gingivalis is the bacterium's ability to subvert
   the roles of organ specific inflammatory cells via a number of
   virulence factors, the most important being its lipopolysaccharide
   (LPS) and gingipains (Singhrao et al., [54]2015; How et al., [55]2016).
   Once in the new niche, P. gingivalis induces dysbiosis of local
   commensals (Harding et al., [56]2017) and drives immune reactions in
   favor of inflammatory amplification whilst maintaining chronic disease
   (Olsen et al., [57]2017). The inflammatory contribution from
   periodontitis links to CVD (atherosclerosis), T2DM and AD and other
   pathologies. The American Heart Association (AHA) declared, after an
   extensive review of the literature, that periodontal disease was
   independently associated with arteriosclerotic vascular disease (ASVD)
   (Bale et al., [58]2017). Similarly, T2DM has been recognized as a
   complication of periodontal disease (Löe, [59]1993) and periodontitis
   has been proposed as a risk factor for AD (Kamer et al., [60]2015;
   Harding et al., [61]2017; Leira et al., [62]2017) with support from
   longitudinal monitoring documented elsewhere (Chen et al., [63]2017).

   AD is associated with impaired cognition and a number of pathological
   lesions, classically amyloid-beta (Aβ) deposits and hyperphosphorylated
   neurofibrillary tangles (Goedert et al., [64]1991). Many pathogens,
   (e.g., herpes simplex type 1 (HSV-1), Chlamydia pneumoniae, Borrelia
   burgdorferi) (Itzhaki et al., [65]2016) as well as P. gingivalis
   lipopolysaccharide (Wu et al., [66]2017) are able to promote Aβ
   deposition. Aβ has broad-spectrum antimicrobial effects against
   bacteria, fungi and viruses (Soscia et al., [67]2010; White et al.,
   [68]2014; Bourgade et al., [69]2015; Kumar et al., [70]2016) and its
   deposition may result from an innate immune response to the cerebral
   invasion of pathogens that have been associated with AD.

   T2DM is a metabolic disorder characterized by hyperglycemia and insulin
   resistance. The complications of diabetes result from long-term
   elevation of blood glucose levels. Diabetes has significant impact on
   gingival and periodontal tissues due to poor glycaemic control (Löe,
   [71]1981). A consequence of hyperglycaemia is that free sugars
   circulating in the blood give rise to advanced glycation end-products
   (AGEs), which are a unique inflammatory product. Endothelial cells and
   monocytes have receptors that bind AGE products. This in turn has an
   impact on gingival tissue vascular permeability through enhanced
   breakdown of the periodontium by protease activity of polymicrobial
   infections (Embery et al., [72]2000; Sugiyama et al., [73]2012). T2DM
   is a well-established risk factor for stroke (Chen et al., [74]2016)
   and as a complication of periodontal disease (Löe, [75]1981). As with
   AD, numerous (and similar) oral pathogens have been associated in
   patients with diabetes (Castrillon et al., [76]2015). The pancreatic
   amyloid (amylin) accumulates in pancreatic islets in T2DM. Amylin also
   has antimicrobial effects and its accumulation may result from an
   innate immune response to chronic bacterial infections, ultimately
   triggering the inflammatory pathology related specifically to T2DM
   (Miklossy and McGeer, [77]2016).

   Periodontitis is associated with age-related diseases such as
   atherosclerosis via immune processes leading to dyslipidaemia in the
   vessel walls (Libby et al., [78]2002; Velsko et al., [79]2014). These
   events follow from established periodontitis (Socransky et al.,
   [80]1998). One feature of the polymicrobial infection is that P.
   gingivalis secretes a peptidyl arginine deiminase enzyme that can
   modify proteins prevalent in atherosclerotic lesions by citrullination
   (Janssen et al., [81]2013; Sokolove et al., [82]2013;
   Geraldino-Pardilla et al., [83]2017). Gearldino-Pardilla et al.
   reported that higher levels of autoantibodies against citrullinated
   proteins can target citrullinated histone 2B associated with higher
   coronary artery calcium scores (amount of calcium in walls of arteries
   supplying heart muscle) when compared with lower antibody levels,
   suggesting a potential role of seroreactivity to citrullinated histone
   in atherosclerosis (Geraldino-Pardilla et al., [84]2017).

   All of the aforementioned diseases have both genetic and environmental
   components, the latter often related to pathogens, as is the case for
   AD (see above), atherosclerosis (Sessa et al., [85]2014), and T2DM
   (Chakraborty et al., [86]2017). Previous studies have shown that host
   genes utilized by oncogenic viruses relate to cancer susceptibility
   genes (Rozenblatt-Rosen et al., [87]2012) and several genes related to
   AD are involved in the life cycles of the many pathogens implicated in
   this disorder (HSV-1, C. pneumoniae, Cryptococcus neoformans, B.
   burgdorferi, Helicobacter pylori, and P. gingivalis) (Carter,
   [88]2011). The HSV-1 and the parasite Toxoplasma Gondii—host
   interactomes also overlap with the susceptibility genes of a variety of
   neurological and psychiatric diseases (Carter, [89]2013a,[90]b). A
   further study of 110 viruses has shown host/pathogen and host/host
   interactome overlaps that are relevant to the genetics of multiple
   human diseases (Navratil et al., [91]2011). These results support the
   notion of important relationships between the genes and proteins used
   by pathogens and disease susceptibility genes. These gene/environment
   interactions are likely to affect disease manifestation.

   In this study, we have compared the P. gingivalis/host interactome with
   GWAS susceptibility genes involved in AD, T2DM and related metabolic
   (obesity) syndromes, and atherosclerosis (CVD, strokes). We have also
   compared the gene expression profiles derived from periodontitis
   gingival tissue or P. gingivalis-treated macrophages with those derived
   from AD hippocampal tissue or atherosclerotic plaques from clinical
   samples.

Methodology

   The P. gingivalis/host interactome (currently comprised of 3,993 host
   genes) curated manually, from Pubmed references is available at